[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A diesel fire pump is rated 3 785 L/min (1 000 gpm) at 6.9 bar (100 psi) net, 1 760 rpm. During the acceptance test the engine ran at 1 780 rpm, and at the 150 % point the net pressure read 4.69 bar (68 psi) at 5 754 L/min (1 520 gpm).

• What are the pass criteria at churn, 100 % and 150 %?
• How should readings be corrected for speed?
• Did this pump pass?

[MEP Center Engineering]

Under NFPA 20 the pump must deliver at least 65 % of rated net pressure at 150 % of rated flow, and churn (zero flow) pressure must not exceed 140 % of rated. Corrected to rated speed, this pump delivers about 1 503 gpm at 66.5 psi, which is above the 65 psi minimum, so it passes at the 150 % point.

Pass criteria (NFPA 20)

Point	Flow	Net pressure requirement
Churn	0	≤ 140 % of rated: ≤ 9.65 bar (140 psi)
Rated	3 785 L/min (1 000 gpm)	≥ 6.9 bar (100 psi)
Overload	5 678 L/min (1 500 gpm)	≥ 65 % of rated: ≥ 4.48 bar (65 psi)

Net pressure is discharge pressure minus suction pressure, both corrected for gauge elevation. Compare the field curve with the certified shop curve, not only with these three minimums. NFPA 25 asks for investigation when annual results fall more than 5 % below the acceptance or nameplate values.

Speed correction (affinity laws)

Diesel engines rarely run at exactly rated speed, so correct the readings: Q_r = Q × (N_r/N), P_r = P × (N_r/N)².

• Flow: 1 520 × 1 760 / 1 780 = 1 503 gpm (5 689 L/min).
• Pressure: 68 × (1 760 / 1 780)² = 68 × 0.9776 = 66.5 psi (4.58 bar).
• Result: 66.5 psi ≥ 65 psi at ≥ 1 500 gpm, so it passes. Do the same correction at churn and at 100 %.

Test arrangement

• A flowmeter loop or a test header with 65 mm (2½ in) hose valves. The header must pass at least 150 % of rated flow. One hose valve per 946 L/min (250 gpm) is the usual basis, so about six for this pump.
• Calibrated gauges on suction and discharge, and a tachometer.
• Suction: at 150 % flow the suction pressure must not fall below the NFPA 20 minimum (0 bar gauge at the flange in general, with a small negative value allowed from a suction tank). Water utilities often set a higher minimum on the main.
• Record motor current and voltage for electric pumps (no overload at 150 %), and engine temperatures for diesel pumps.

EN 12845 note

EN 12845 also requires a stable, non-overloading pump curve, with a drive rated for the maximum power along the curve, and its own flow test arrangements. Two pumps are required for superior or duplicate supplies, and they are separate pumps: a duty pump and an independent standby pump, each with its own driver. Never use a twin-pump unit for this.

Common mistakes

• Judging the pump on discharge pressure without subtracting suction pressure.
• Skipping speed correction on diesel pumps.
• A test header too small to reach 150 %, so the overload point is never tested.

Room layout and test lines: Fire Pump Room Design Checklist.

For deeper study: Fire Protection Design course.

Check the edition of the code that applies to your project.

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